The booby prize this year for Dirtiest City in America goes to Fresno, California. This Central Valley city suffers some of the worst air in the nation, and a water supply so degraded that the city used to tell pregnant women not to drink from the tap. Fresno epitomizes the environmental challenges of the Golden State. And it's not alone. Plenty of its neighbors in central California like Modesto, Stockton and Bakersfield have it almost as bad. (Other California metro areas ranked among the 20 Dirtiest Cities include San Jose, Riverside and Los Angeles -- but don't worry, they have lots of company from towns in the Midwest and on the East Coast, too.)

The environmental degradation of the Central Valley has many contributing factors. First of all, its geography doesn't do it any favors. It's a big, long bowl surrounded on three sides by mountains that trap pollutants from cars and factories and oil fields in an inversion layer.

Second, it's a victim of what brought people there in the first place -- rich fertile soils from which grow much of America's fruits and vegetables. For decades farmers would burn leftover cuttings from their fields after the harvest -- dumping massive amounts of lung-choking particulate matter into the air. Burning has been banned since 2004, and the air has gotten cleaner since then, but there's still a long way to go. Compounding the problem are frequent dust storms, exacerbated by water shortages.

Third, all that agricultural activity has wreaked the Valley's groundwater. Decades of fertilizer and pesticide applications and manure from livestock have caused noxious chemicals to trickle down into the water table. When pumped back up into homes, the water regularly gives people rashes when they shower in it. Nitrates in the water can cause babies who drink it to come down with potentially fatal "blue baby syndrome."

What's troubling is that rather than getting better, the water problem might be getting worse.

An EPA report on California's waters last year found the number of rivers, streams and lakes with meaningful levels of toxicity rose more than 170% from 2006 to 2010. Of the 215,000 miles of shoreline, streams and rivers in the Golden State, 30,000 miles are not meeting water quality goals, and 20,000 need a pollution cleanup plan. What's more, says the EPA, of California's 3.0 million acres of lakes, bays, wetlands and estuaries, 1.4 million acres do not meet water quality goals and need to be cleaned up.

Unfortunately, in much of the Central Valley, the water pollution is so pervasive that that there's virtually no possibility of rehabilitation. According to a study for California by U.C. Davis Prof. Thomas Harter, remediating the water in the Tulare Lake Basin and Salinas Valley would cost around $30 billion a year over 20 years.

As if all that's not enough, Fresno also is home to Superfund sites like the Fresno Municipal Landfill -- which has leaked chemicals like vinyl chloride into groundwater -- as well a pesticide factory that leaked so much gunk into groundwater that nearby residents had to be provided with an alternative water supply.

Depressed yet?

Well help may be on the way.

Last month California held its first auction for carbon allowances under the state's new
cap-and-trade law that will force any company that emits more than 25,000 tons of carbon dioxide a year to reduce its emissions, or pay the price for permits. Though carbon dioxide itself doesn't contribute much, if at all, to air quality problems, the other emissions that often accompany it out of smokestacks, like sulfur dioxide and soot, certainly do.

Starting in 2015 California's oil refineries will also become responsible for the emissions from cars that use their fuel. This is a cost that they will, of course, pass along to motorists in the form of higher gasoline prices. But to the extent that Californians vote with their wallets and buy more fuel-efficient cars, the effect should be to reduce air pollution in the state. (It would also likely drive refiners and other big emitters out of California entirely, but that's a different discussion.)

Estimates are that for every $10 a ton in the price of carbon allowances, California gasoline prices will go up a dime. The first cap-and-trade auction resulted in a price of $10.09 per ton. If the scheme works in California, it might not be long before our intrepid lawmakers in Washington, D.C. attempt to do the same thing nationwide.

A bright spot in the pollution landscape is that according to this EPA report, America's air quality has generally been getting better. Cars and trucks are more efficient, fuel blends are cleaner, and coal-fired power plants have been forced to install air-scrubbing technology.

But lord knows we have a long way to go.

Much of the data for our list of the 20 Dirtiest Cities In America is derived from rankings done by Bert Sperling on his Bestplaces.net website. Sperling is known for for his books like "Best Places To Raise Your Family" and "Cities Ranked And Rated." His website features tools that let you slice and dice cities by a whole host of categories, like cost of living and income, as well as by water and air quality.

Sperling's air quality index is based on annual data from the EPA, which factors in ozone and pollutants like carbon monoxide, sulfur dioxide, lead and volatile organic chemicals.

Likewise, his water quality rankings use EPA data that measures watershed quality by looking at 15 indicators like pollutants, sediments, and toxic releases.

Both indices rank metropolitan areas into an index where 1 is the worst and 100 is the best.

To get a set of dirty cities, we identified which metro areas (with population of 500,000 or more) showed up on both lists for poor water quality and air quality. Then we averaged their rankings on the indices. We then took a look at some other sources, such as EPA data on the big cities with the most toxic releases (Houston, Texas -- ranked 13th -- leads by a mile), the American Lung Association's annual State of the Air report, as well as a look at the prevalence of Superfund sites in the various areas.